Method and apparatus for effecting high-energy dynamic coating of substrates

ABSTRACT

An electrical plasma-jet spray torch is adapted to effect spray coating of substrates in a reduced-pressure chamber, at supersonic plasma velocities, thereby achieving extremely dense coatings of high-purity material. The spray powder is preheated to a predetermined temperature before entering the plasma, and is delivered simultaneously to the plasma from a plurality of powder sources. Both the plasma and a transferred arc are employed to preheat the substrate to a predetermined temperature, before the spraying operation commences. Furthermore, the plasma is employed to effect post-annealing and stress-relieving functions. The transferred arc power source is connected between the plasma torch and the substrate, to add energy to the powder while in flight and to provide a fusion bond between the coating and the substrate. The gaseous environment in the spray chamber, and in the powder-inlet passages, is carefully selected and controlled to achieve desired results.

United States Patent 1 91 1111 3,839,618

Muehlberger 1 Oct. 1, 1974 [54] METHOD AND APPARATUS FOR 3,360,682 l2/l967 Moore 219 121 P EFFECTING HIGH ENERGY DYNAMIC 3,598,944 8/1971 Weiman 01 al 219/76 COATING OF SUBSTRATES Primary Examiner-Bruce A. Reynolds Inventor: Erich Muehlberger Costa Mesa Attorney, Agent, or Firm-Allan Rothenberg; Richard Cahf' L. Gausewitz [73] Assignee: Geotel, lnc., Long Island, N.Y.

22 F1 d A 17 1973 [57] ABSTRACT 1 1e An electrical plasma-jet spray torch is adapted to ef- [21] Appl. No.: 351,814 fect spray coating of substrates in a reduced-pressure Related Us Application Data chamber, at super-sonic plasma velocities, thereby 63 f S N 214 I achieving extremely dense coatings of high-purity macommuanon'm'part 0 terlal. The spray powder 1s preheated to a predeter- 1972 mined temperature before entering the plasma, and is delivered simultaneously to the plasma from a plurall52] US. Cl..... 219/121 P, l17/93.l PF, 117/21l%5/.71 ity of powder Sources Both the plasma and a trans 5 1] Int Cl B23k 9/04 ferred are are employed to preheat the substrate to a 58] Fieid P 75 7O predetermined temperature, before the spraying oper- H7293 1 i b atlon commences. Furthermore, the plasma 15 employed to effect post-annealing and stress-relieving References Cited functions. The transferred are power source is connected between the plasma torch and the substrate, to

UNITED STATES PATENTS add energy to the powder while in flight and to pro- 3,010,009 11/1961 Ducati 219/76 vide a fusion bond between the coating and the sub- 3,07l,678 l/l963 Neely ct al 219/76 strate. The gaseous environment in the spray chamber, 3.1791782 4/1965 Millmy A 4 3 219/76 and in the powder-inlet passages, is carefully selected 3,179,783 4/1965 Johnson ..2l9/76 t e qird Slt 3,182,361 5/1965 Trimble ..219/76 con ed oac e 3,183,337 5/1965 Winzelcr 219/76 45 Claims, 3 Drawing Figures METHOD AND APPARATUS FOR EFFECTING HIGH-ENERGY DYNAMIC COATING OF SUBSTRATES CROSS REFERENCE TO RELATED APPLICATION This application constitutes a continuation-in-part of my copending patent application Ser. No. 214,584, filed Jan. 3, 1972, for an Apparatus and Method for Plasma Spraying, now abandoned and containing subject matter incorporated in a continuation application, Ser. No. 372,260, filed June 21, 1973, and the disclosure thereof is disclosures incorporated by this reference as though fully set forth herein.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the field of methods and apparatus for effecting plasma spray-coating of various substances onto substrates.

2. Description of Prior Art Plasma-spray coating of substrates in reducedpressure chambers is taught by Ducati US. Pat. No. 3,010,009, issued Nov. 21, 1961. However, such patent does not disclose super-sonic plasma velocities, nor powder or substrate preheat, nor various other important matters which permit achievement of surprisingly dense, pure, and bonded coatings.

Plasma spraying at super-sonic plasma velocities has been effected in the atmosphere, for more than a year prior to the filing date of the present application, as taught by my copending patent application Ser. No. 143,956, filed May 17, 1971, now abandoned, for Coating Heat Softened Particles By Projection in a Plasma Stream of Mach 1 to Mach 3 Velocity, and

also by my copending patent application Ser. No. 133,126, filed Apr. 12, 1971, now Pat. No. 3,740,522,

for a Plasma Torch, and Electrode Means Therefor.

The disclosures of both of these patent applications are incorporated by this reference as though fully set forth herein. The use of super-sonic wind tunnels of the plasma type for chemical synthesis and other purposes, is taught by US. Pat. No. 3,360,682, issued Dec. 26, 1967.

Relative to preheating of the spray powder prior to introduction into plasma employed for coating a substrate, applicant knows of no prior art. Patent No. 3,598,944 relates to heating of particulate material before it is introduced into a plasma heating zon in a device for creating spherical granules of nuclear fuel (as distinguished from in a plasma spray torch). Preheating the substrate in an inert atmosphere at atmospheric pressure has been described for achieving metallurgical bonding of tungsten coatings on polished tungsten substrates.

The use of a plurality of power sources, for spraying at sub-sonic plasma velocities, is taught by Pat. No. 3,183,337, issued May 11, 1965. The connection of an are power source between the torch and workpiece, in plasma spraying at atmospheric pressure, is taught by Pat. No. 3,179,783. Prior transferred arc plasma guns have relied solely on thermal effects of plasma and of the transferred are for providing a hard surfacing. This has been achieved by melting the substrate together with an additional material in the form of a welding rod or powder over a small, localized spot of high temperature and to a relatively great depth below the surface of the substrate.

SUMMARY OF THE INVENTION The present method and apparatus make possible and practical the spray-coating onto various substrates of numerous metals, alloys, oxides, etc., to create highpurity coatings the densities and bond strengths of which approach theroetical maximums. The method and apparatus relate to the directing into a reducedpressure environmental chamber of a plasma jet wherein the plasma velocity is in the range of Mach 1 to Mach 5, introducing spray powder into the jet, and causing the spray powder to soften and then impinge against a substrate. The pressure in the environmental chamber should be below one-half atmosphere and is preferably very .much lower. Although the chamber pressure is thus low, the stagnation pressure immediately adjacent the substrate is high, and this aids in creating dense and well-bonded coatings. Several methods are employed to impart additional heat energy to the particles.

Before the introduction of spray powder is effected, the plasma jet is directed against the substrate for a time period sufficient to preheat the substrate to a desired temperature. Furthermore, for spraying of many types of powders, an arc power source is connected between torch and substrate, thus adding heat energy to the particles as they fly toward the substrate. The energy thus added may bemade sufficiently great to effect localized fusion at the substrate surface, thereby enhancing the quality of the bond between substrate and coating. The are power source also may be used for preheating of the substrate.

The spray powder particles are of small size, and are preheated. to a predetermined temperature before reaching the plasma. Such preheating is accomplished by means of an electrical-resistance tube through which the powder and its carrier gas are passed. The hot carrier gas is adapted to create desired chemical effects, for example reducing effects, on the spray powder. Furthermore, the composition of the plasmaforming gas may be such that desired chemical effects occur in the environmental chamber.

Spray powder is delivered to the plasma from a plurality of sources which operate simultaneously. The powder and its gas should be introduced into the supersonic nozzle of the plasma torch in the region of the nozzle throat. The direction of powder introduction is prefereably upstream, to increase the dwell time of the spray particles in the jet. In certain arrangements, the powder introduction conduit extends through the nozzle wall at an acute angle relative to the surface of the nozzle passage.

After the spray powder is turned off, the plasma jet is continued in operation for a time period sufficiently long to post-anneal the coating, thereby relieving thermal and other stresses.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of the apparatus for effecting high-energy dynamic coating of substrates;

FIG. 2 is a longitudinal central sectional view of the plasma torch portion of the coating apparatus; and

FIG. 3 is a transverse sectional view on line 3-3 of FIG. 2.

DETAILED DESCRIPTION OF THE'PREFERRE EMBODIMENT I As stated in my copending application for Coating Heat Softened Particles by Projection in a Plasma Stream of Mach 1 to Mach 3 Velocity, Ser. No. 143,956, it has been known that coating qualities including characteristics of bonding strength, coating density and coating uniformity show marked improvement with increasing velocity of the impinging particles. This is due in part to the fact that the mechanical working effected by impingement of such high velocity particles enhances the bonding. Accordingly, as set forth in such application, Ser. No. 143,956, the powder to be spray coated is entrained in a super-sonic stream of plasma so as to maximize particle velocity and thus achieve the highly desirable qualities resulting from such high velocity. However, for certain particles, and in particular for those requiring high heat inputs to attain the desired near-melted (but not molten) state, certain characteristics inherent in the high-velocity mechanism create obstacles to optimum spray coating of such particles at high velocity. High velocity of the super-sonic plasma stream works to cool the particles that are initially heated within the plasma torch nozzle, or at least to significantly decrease the continued transfer of heat to these particles during the flight from the nozzle to the workpiece substrate. As is well known, the super-sonic stream is rapidly expanded to attain its high velocity and, concomitantly, its temperature drops significantly. In fact, in some high-velocity plasma torches, temperature in the arc chanber may be as high as 25,000 F. whereas temperature of the are after it has passed some distance from the nozzle exit, may drop by a factor of two or three so that the plasma arc may have a temperature in the order of only l2,000 F. as it nears the workpiece. Since super-sonic spray coating involves injecting particles into the plasma stream within the nozzle at a point within or close to the nozzle throat, the heating effect upon the particles due solely to temperature difference is maximum within the nozzle. As the plasma stream expands and cools, this temperature difference decreases and the heat imparted to the particles likewise decreases. Accordingly, in some situations, although the particles to be spray coated may acquire the desired high kinetic energy, it is possible that they may not be heated to the desired value on impingement at the substrate.

A significant feature of the present invention comprises methods and apparatus for adding thermal energy to particles to be spray coated at very high velocity. In accordance with one feature of the invention, the

particles are preheated in elongated powder injection tubes as they are being transported to the nozzle and to the plasma stream therein. In accordance with asecond feature of the invention, heat energy is added to the particles after they leave the nozzle and while they are in flight. This is in addition to the heatenergy that is added by the plasma stream inwhich the particlesare entrained. The added heat is imparted to the particles in flight by means of a transferred are that flows an electrical current from the nozzle to the substrate. The transferred arc flows both through the plasma and through the particles entrained therein during flight of the particles. It provides electrical heating of the particles while they are in flight.

The use of the transferred arc for this additional heating of high kinetic energy particles has a number of additional advantages, all of which combine to improve the spray coating process. The transferred arc is a considerably more efficient source of heat for the substrate as compared with the heat provided to the substrate by the impinging plasma stream. Approximately only 30 percent of the plasma are power source is transferred by the plasma arc to the substrate as heat, whereas percent or more of the transferred arc power source is transferred to the substrate as heat.

' At least in part due to the very high efficiency of heating and the rapid heating effect of the transferred arc, the heat introduced into the substrate thereby is concentrated at the surface of the substrate where the actual bonding takes place. This effect is even further enhanced when the process is employed for spray coating of a substrate contained in a low pressure or substantially evacuated chamber. In such a case, the plasma arc, together with the particles therein, are diffused, spreading over a relatively larger area and may impinge upon substrate area as great as five inches in diameter when the latter is sixteen inches from the plasma torch, for example. As the plasma stream and entrained particles are diffused, so too, the current path of the transferred arc is diffused. Accordingly, the transferred arc will impinge upon an enlarged area of the substrate, substantially equivalent to the area of impingement of the plasma stream. This diffusing effect further concentrates the transferred arc heating at the substrate surface, and avoids any significant deep penetration of the heating effect below the substrate surface. The arrangement substantially eliminates localized spot puddling effects of the prior art wherein melting of the substrate occurs to significant depth.

The above-described features of the transferred arc, namely the faster and more efficient heating of the substrate and the diffusedor enlarged substrate heating area additionally provide for significantly increased speed of preheating of the substrate. Accordingly, use of the transferred arc allows much more rapid heating of larger areas of a massive substrate. It does not require heating of the entire substrate (in depth) whereby it is possible to keep the back side or other portions ofa substrate at a relatively low temperature even though the surface thereof to be coated is heated to temperatures sufficient to achieve a proper metallurgical bond.

Referring now to FIG. 1, an exemplary apparatus for carrying out principles of the present invention comprises an elongated tank 10 which defines a sealed environmental chamber 11 containing a substrate 12 to be spray-coated. In the illustrated apparatus, substrate 12 is disposed in a vertical plane transverse to the longitudinal extent of tank 10, being mounted by a bracket 13 on a truck 14 which rolls along a track 16. The track 16 is oriented longitudinally of the tank 10, so that the gate valve 20, and a filter 21 to the environmental chamber 11. The pump means 18 exhaust to the atmosphere, or may be adapted to recirculate gas back to the plasma torch 17. Pump means '18 may incorporate a diffusion pump and a mechanical pump, in seriesrelationship to each other, the diffusion pump being upstream relative to the mechanical pump.

Filter 21 is adapted to remove from the gas emanating from chamber 11 any particles of spray powder or other material which do not adhere to the substrate 12. Mounted upstream from filter 21, between such filter and the substrate 12, is a baffle plate 22 adapted to block excessively high-velocity flows of gas into'the filter. Baffle plate 22 may be cooled by suitable means, for example water-cooling coils, not shown.

The plasma torch 17 is mounted in a sealed reentrant outer housing 23' which is sealingly related to the end wall of tank 10. The entire outer housing 23 is shown in FIG. 1, and the right end portion of the outer housing 23 is shownin FIG. 2. The plasma torch 17 is illustrated as being of the general type described in detail in my copending patent application Ser. No. 133,126, and in my copending application Ser. No. 214,584, cited above, the specifications and drawings of which are incorporated by reference herein as if set forth in full. Thus, the torch 17 comprises two annular housing elements 24 and 25 formed of synthetic resin, and which nest with each other and with an annular front housing element 26 of highly conductive metal. A rear electrode assembly 27 projects forwardly through housing elements 24, 25, terminating at the front end thereof in a tungsten slug 28. Tungsten slug 28 is disposed concentricaly of a super-sonic nozzle element 29 which is formed of metal and is seated coaxially in the front housing element 26.

Nozzle element 29 has a relatively large-diameter rear portion 31 in which the tungsten slug 28 is concentrically disposed, and also has a somewhat smallerdiameter central portion 32 adapted to effect seating of the forward foot point of the electric arc. Portion 32 communicates coaxially with the cylindrical portion 33 which constitutes the throat of the nozzle, the portion 33 having a diameter smaller than that of the central portion 32. Th forward end of throat 33 communicates with a forwardly-divergent conical portion 34 which communicates with the environmental chamber 11.

The divergent portion 34, and a part of throat or cylindrical portion 33, are disposed in a forwardlyprotuberant region of element 29 which extends forwardly of front housing element 26, being sealingly associated with the cylindrical wall of an opening in outer housing 23. Furthermore, portion 36 is adapted to provide an entrance region for spray powder as stated in detail hereinafter. The nozzle is arranged to provide the convergent-divergent configuration for producing the expanded super-sonic plasma flow to be described below.

The tungsten slug 28 is connected coaxially to a copper base 37 which, in turn, is threadedly related to a slug holder portion 38 of the rear electrode assembly 27.

Means are provided to introduce arc gas around copper base 37 and thence forwardly around slug 28, for flow through the nozzle portion 31, 32, 33 and 34 into the chamber 11. This comprises a suitable source 39 of high-pressure gas and which is connected through a conduit 41 to an annular chamber 42 which surrounds an annular gas-injector ring 43 formed of ceramic or other heat-resistant insulating material. The conduit 41 is illustrated schematically only, since it actually extends through housing elements 24 and 25 in the man ner described in the cited patent application Ser. No. 214,584. From chamber 42, the gas flows inwardly through passages in gas-injector ring 43 to the space surrounding the forwardly-protuberant cylindrical portion of slug holder 38, following which it flows forwardly around the copper base 37 and thence around tungsten slug 28. There may be a multiplicity of passages through the gas-injector ring 43, and these may be inclined in various directions, one preferred manner of introduction being tangential in order that the infiowing gas will pass vortically around the slug 28.

A water and current-carrying conduit 45 is provided as shown in the lower portion of FIG. 2, connecting to the front housing element 26. Water flows inwardly through conduit 45 from a suitable water source 46. It then flows upwardly into housing element 46 in encompassing relationship to the super-sonic nozzle element 29. More specifically, the water flows around a generally cylindrical baffle 47, then flows forwardly around the front end of the baffle 47, then flows rearwardly between the baffle and the exterior surface of nozzle 29, then flows rearwardly out the read end of the baffle 47, and then enters a chamber 48 defined between housing elements 25 and 26. Suitable spacer means 49 are provided to mount the baffle 47 in outwardly-spaced rela- .tionship from the wall of the nozzle 29.

From chamber 48, the water flows rearwardly through passage means 51 to an annular groove 52 in rear electrode assembly 27, following which it flows into passage means 53 in the assembly 27 and thence out through a conduit 54 which conducts both water and electricitypThe conduit 54 connects to a suitable drain 55.

Means are provided to feed spray powder, at high rates, to the plasma. More specifically, means are provided to feed powder to the plasma which is still in the nozzle passage in nozzle 29. Thus, bores 56 and 57 are provided in diametrically opposite portions of nozzle element 29 as shown in FIG. 2, the outer end of each bore being plugged, the inner end being in communication with throat 33 of the nozzle passage. The bores 56 and 57 are inclined relative to the axis of the torch and extend at a relatively small acute angle with respect to the surface of the conical divergent portion of the nozzle passage. This arrangement causes the powder to be introduced into the nozzle throat, flowing in a generally upstream direction relative to the plasma, whereby to increase the dwell time of the spray powder in the plasma. Introduction of the powder into the nozzle throat accomplishes maximum transfer of kinetic energy to the powder because the plasma has maximum density at the throat. Bores 56 and 57 communicate, respectively, with passages 58 and 59 (FIGS. 2 and 3) which, in turn, connect to elongated electrically conductive powder-conductor tubes 60-61.

The powder tubes 60-61 are quite long, and pass through sealed couplings 62 (FIG. 1) which permit passage through the wall of the sealed tank 10. The tubes 60-61 communicate, respectively, with powder feed sources 63-64. Each source 63-64 may be of the general type described in U.S. Pat. No. b 3,517,861 for Positive-Feed Powder Hopper and Method.

Referring to FIGS. 2 and 3, the ends of powder tubes 60-61 are electrically and mechanically connected to the exterior of nozzle element 29 by first. andfsecond semicircular split-ring elements 66-67. Each split ring 66-67 is preferably formed of metal, and has a metal coupling element 68 fixedly secured therein, as by press-fitting and/or brazing. The outer ends of element 68 connect to powder tubes 60-61. The inner ends of element 68 are beveled and seat forcibly against conical connecting portions of passages 58-59 to couple the split rings 66-67 to nozzle 29, with the inner ends of coupling element 68 seated in the conical connecting portions of passages 58-59. Screws 69 are inserted through opposed portions of the split rings and are suitably tightened to secure the ring and tube assembly to the nozzle.

The simultaneous use of a plurality of sources of spray powder and spray-powder carrier gas, isalso shown and disclosed in the above-cited patent application Ser. No. 214,584.

Proceeding next to a description of the various electrical connections in the system, an arc power supply is schematically represented at 71, having its negative terminal connected to conduit 54 and its positive terminal connected to conduit 45. Thus, the negative terminal of source 71, which is a DC power source, connects to the rear electrode assembly 27 and thus to tungsten slug 28. Correspondingly, the positive terminal of source 71 connects to the front housing element 26 and thus to the super-sonic nozzle element 29 which also operates as the anode of the torch.

The present system also incorporates a high-current DC power supply 72 (FIG. 1), which constitutes the transferred-arc power supply and is connected between nozzle element 29 and the substrate 12. Staged more definitely, the negative terminal of power supply 72 is connected to the positive terminal of arc power supply 71 and also to conduit 45. The positive terminal of source 72 is connected to substrate 12, the connection being through a coupling 76 and flexible conduit 74 (FIG. 1) and also through the truck 14 and bracket 13.

The apparatus further comprises a powder preheat power supply 76, which is a DC source the negative terminal of which is connected to each of the two electrically-conductive powder tubes 60-61, preferably at or near outer ends thereof. The positive terminal of source 76 connects to the negative terminal of source 72 and to the positive terminal of source 71.

DESCRIPTION OF THE METHOD The first step in the method comprises operating the pump means 18 in order to reduce the pressure in environmental chamber 11 to a value far below atmospheric. The pressure in chamber 11 should be below one-half atmosphere, and is preferably caused to be a very small fraction of atmospheric. For example, the pressure in chamber 11 may be reduced to 150 microns of mercury before the are power and are gas are turned on.

The truck 14 is moved to properly position the substrate 12. The distance between the substrate 12 and the protuberant portion 36 of the nozzle 29 may be on the order of about 16 inches, although other distances may be employed; A precise distance may be determined empirically in order to achieve optimum spray conditions.

After the chamber is thus substantially evacuated, the arc gas source 39 and are power source 71 (FIG. 2) are turned on and an arc is started between tungsten slug 28 and the wall of nozzle portion 32. Starting may be by high frequency or other means. The are current is caused to be very high, for example 800 amperes, at a voltage of volts. The gas delivered from source 39 is preferably argon or helium, although various other gases may be employed, for example to provide desired environmental conditions in the chamber 11. The pressure in the arc chamber is high, for example 7 atmospheres.

The vacuum pumping is continued throughout the process to maintain a pressure of about 40 mm of mercury within the environmental chamber 11.

The described high-current electric arc, gas flow, and are chamber pressure cooperate with the super-sonic nozzle and the low pressure in chamber 11 to create a very long and rapidly-diverging super-sonic plasma jet which is indicated at 77 in FIG. 1. The jet 77 will cover a relatively large area of substrate 12, which operates (in combination with the rapid introduction of spray powder, as described below) tov effect rapid formation on the substrate of a layer of coating material.

The plasma torch 17 is operated to direct the jet 77 against substrate 12 for a time period sufficiently long to effect preheating of the substrate 12 to a desired temperature. Such temperature, and the duration of the preheating step, are determined empirically for various substrates, various powers, etc. It is emphasized, however, that the use of the plasma jet 77 to effect preheating of the substrate cuases' the preheating operation to be accurate, effective and fast.

The substrate preheating operation is preferably augmented by turning on the transferred are power supply 72, which applies a voltage between the nozzle 29 and the substrate 12. The result is the flow of high current through the plasma 77, which acts as a conductor, resulting in a corona discharge around the substrate 12 and adding energy to the system whereby preheating occurs more rapidly than would otherwise be the case. The heating effect ofthe transferred arc provides additional advantages. It is more efficient, since as much as 90 percent of the transferred are power source may appear as heat at the substrate whereas only about 30 percent of the arc power source is transferred as heat to the substrate by the plasma stream. Furthermore, the heating effect of the transferred arc is concentrated at the surface of the substrate where the bonding is to take'place. Thus, a large massive substrate need not have all of its mass heated to proper bonding temperatures.

At an appropriate time during the preheating step, the powder feed sources 63-64 are caused to supply powder-conducting gas (but not the powder itself) to tubes 60-61 and thus to the plasma. The composition of the gas may be argon, helium, etc., although it is emphasized that the invention comprises employing gas which will adapt to predetermined chemical reactions with the powder being supplied to the torch. As an example, the gas passed through tubes 60-61 may be hydrogen, in order to effect reduction of oxides which are supplied from sources 63-64 to the torch.

After the flow of powder-conducting gas (but now powder) through tubes 60-61 has been initiated, the powder preheat power supply 76 is applied to thus effect electrical-resistance heating of tubes 60-61. Such tubes are formed of electrically resistive material, for example stainless steel or inconel, and are heated to high temperatures ranging up to 1,000 2,000 F. In an exemplary embodiment, each tube is about 6 feet in length, having an inside diameter of about onesixteenth of an inch and a wall thickness of 0.030 inches. The temperatures of tubes 60-61 are monitored by means of suitable thermocouples (chromel-alumel) or by an optical pyrometer (not shown). The thermocouples, when employed, should be fastened to one or both of tubes 60-61 a short distance (for example about 1 inch) from the split rings 66-67.

After the substrate preheating has been effected, and after the powder tubes 60-61 are at the desired temperatures and there is powder-conducting gas flowing therethrough, then the powder sources 63-64 are operated to deliver powder to the powder-conducting gas so that spray powder flows through tubes 60-61 to the plasma. Such spray powder is first preheated in the tubes 60-61, then is introduced into the plasma, preferably by passing into the throat of the nozzle element 29 through passages 56, 57 in the upstream directions described relative to FIG. 2, and then is further heated both by the plasma stream and by transferred arc current while traveling with the jet 77 to the substrate 12. The combined heating steps cause the powder particles to be brought to a high temperature which should be just below their melting points, so that the particles are soft but are not molten. The particles are thus in a near molten state and at nearly maximum velocity in flight within the projected plasma stream at a point between the nozzle exit and the substrate. The particles thus impinge against substrate 12 in soft condition and at great velocities, which is an important factor tending to cause formation of a highly dense coating of powder material on the substrate 12. It is pointed out that different powders may be employed in sources 63-64, in order that alloying will occur as desired. Although the described powder injection angle and position is preferred for maximized transfer of kinetic energy, other powder injection combinations may be employed for use with particles requiring less heating.

The velocity of the plasma emanating from the torch is very high, for example 13,000 feet per second. Furthermore, the spray powder is in the plasma for a long distance, for example 16 inches. Although the spray powder does not move nearly as fast as does the plasma iteslf, the spray powder is nevertheless accelerated to high velocities which (when combined with the soft condition of the particles) cause a very dense coating to occur on the substrate 12. As indicated in the specific examples described below, the method and apparatus of this invention will form non-porous, high density coatings metallurgically bonded to the substrate.

The particle size of the powder fed to the torch is small. The maximum diameter of each particle should not be above 44 microns and is preferably much less.

When the substrate 12 is an electrical conductor, the transferred arc power supply 72 may be employed to increase the energy imparted to the particles and, if desired, to cause a localized fusion to occur between the inner surface of the coating and the outer surface of the substrate. Power supply 72 may deliver a very high current, for example 100 amperes at 130 volts, which current becomes smaller when electrically conductive powder is introduced into the plasma jet 77.

Although the pressure in environmental chamber 11 is maintained at a lowvalue, it is emphasized that there is a high stagnation pressure immediately adjacent the coating on substrate 12. This stagnation pressure may be several atmospheres, and results from a very high velocity of the jet 77 which immediately is reduced to zero as the substrate 12 is struck. The high stagnation pressure against substrate 12 is a factor causing working or hammering of the coating by the gas, thereby tending to increase the density of the coating.

Not only is the coating very dense, but it is caused to be pure due to the environmentally desirable conditions present not only in chamber 11 but in the powder tubes 60-61. Thus, for example, coating can be caused to be substantially percent free of oxides, if desired.

During continuance of the spraying operation, the pump means 18 continues to operate in order to maintain the pressure in chamber 11 at the desired low value. The plate 22 prevents the plasma from entering the filter 21 at an excessive velocity, and the filter 21 removes any excess spray powder from the gas being passed through the gate valve 20 and conduit 19 to the pump means 18 and thus to the atmosphere (or back through the gas-introduction conduit or tube 41 for recirculation to the plasma torch 17). It is pointed out the suitable means may be provided to effect water cooling of the holder or supporbplate for the substrate which is sprayed in accordance with the present method. Such cooling may be employed in conjunction with various types of substrates, particularly those which tend to melt at; relatively low temperatures. Thus, even though part (the back side, for example) of the substrate is cooled, the surface heating added by the transferred arc helps maintain the substrate surface conditions required for good quality coating.

After a coating of the desired thickness is caused to deposit on substrate 12, powder sources 63-64 are operated to shut off the flow of powder to the torch. However, operation of the torch (and the transferred arc, if desired) is continued for a desired time period in order to effect post-heating and heat-treating of the substrate and its coating. The post-heating causes stress relief in the coating, with attendant desired results. The present method therefore comprises the simultaneous and/or sequential spraying and heat-treating of the substrate,

followed by desired annealing steps. As previously mentioned, alloying operations may be effected by simultaneously delivering different powders from the sources 63-64. Because a plurality of powder feed sources 63-64 are'employed simultaneously, the rate of deposition of powder on substrate 12 is greatly augmented.

The various parameters, power, temperature, are and environmental chamber pressures, and nozzle ratio of exit to throat area, are so adjusted that the gas emanating from plasma torch 17, as the plasma jet 77, has a velocity in the range Mach 1 to Mach 5. Desirably, a velocity of Mach 3 is particularly practical to employ.

The present method therefore comprises a highenergy (both kinetic and thermal) dynamic metallurgical process which may be employed to coat numerous conductive and nonconductive substrates 12, for example with various metals and oxides, with numerous types of pure and/or alloyed coatings of metals, ceramics and other materials. The coatings are applied extremely rapidly, to accurately control the thicknesses, and are pure, extremely dense and non-porous. The metallurgical treating steps which are effected both before and after application of powder are factors tending to increase the quality of the finished product. True metallurgicalbonding is accomplished. Specific Examples The following common (to all examples) conditions were employed in each specific example described below. Arc power supply 71 was 800 amps at 90 volts. The work chamber was initially evacuated to 150 microns of mercury and thereafter, the vacuum pumping was continued to maintain a pressure of 40 millimeters of mercury during the preheat and coating process. The arc gas employed was a mixture of 400 standard cubic feed per hour of argon and 107 standard cubic feed per hour of helium. Powder gas was employed at a rate of 50 standard cubic feed per hour of argon. The spraying distance, that is, the distance between the torch exit nozzle and the substrate was 16 inches, plus or minus about 0.25 inches. Example 1 An inner metallic of tungsten carbide/cobalt powder (WC 88 percent and 12 percent Co) having a particle size of from 5 to 20 microns was coated upon a substrate of 410 stainless steel with the above described common conditions. In this example, the substrate was not preheated and no transferred arc power was employed. However, the powder tubes were preheated by supply 76, employing 50 amps per powder heating tube at 30 volts. A coating of mil thickness was achieved in 25 seconds. This was a good, dense coating, but a metallurgical bond was not achieved. Example [I The materials employed in Example I, both powder and substrate, were used with the above-identified common conditions. The substrate was preheated with both thetransferred arc and the plasma jet to a temperature of approximately 2,000 F. Transferred are power from supply 72 was employed at 100 amps and 130 volts. The same powder and powder preheat power was employed as in Example I. It may be noted that the transferred arc current increases as the powder particles are fed into the stream since these particles participate in conducting the trnasferred arc current to the substrate. During preheat, the temperature of the substrate rose to 2,080 F. and then stabilized at 2,050 F. during the coating. A coating of 9 mils thickness was achieved in seconds. Upon analysis, this coating exhibited an exceedingly high density, with no porosity. A metallurgical bond was established. Example III A metal alloy, nickle chromium powder of 15 to 44 microns in size, was sprayed upon a preheated substrate of 304 stainless steel employing the aboveidentified common conditions. Powder tube preheating was not employed. A transferred arc of 90 amps at 130 volts power was employed for assisting (the plasma stream itself) in preheating of the substrate and in enhancing the coating processes. A coating of 12 mils in thickness was achieved in seconds having an exceedingly high density. A metallurgical bond was established. Example lV Employing the above-identified common conditions, particles ofa pure ceramic, aluminum oxide, of 5 to 37 microns in size were sprayed upon a substrate of 410 stainless steel without employing any transferred arc. Powder tube preheating was employed as described in Examples 1 and 11 using 50 amps per tube at about volts, but without preheating of the substrate. A coating of 10 mils in thickness was achieved in 80 seconds. The coating was observed to be slightly grey in color, of high density and without porosity. Electrical resistance of the coating was found to have an unexpectedly high value of 350 volts per mil. This is an unusually high quality dielectric coating having a resistance value considerably higher than many previously attainable.

The foregoing detailed description is to be clearly un- -derstood as given by way of illustration and example only, the spirit and scope of this invention being limited solely by the appended claims.

What is claimed is:

1. Apparatus for effecting spray-coating of a substrate, which comprises:

a. means to define an environmental chamber,

b. means to reduce the pressure in said chamber to a value below atmospheric pressure,

0. an electrical plasma-jet torch positioned to direct a jet of plasma into said chamber, against a substrate disposed in said chamber,

d. means to supply spray powder to said plasma jet for heating therein and subsequent impingment against said substrate, and

e. means to effect preheating of said spray powder to an elevated temperature prior to the time said spray powder reaches said plasma, said preheating means cooperating with said plasma-jet torch to effect a high degree of heating of said powder prior to the time that said powder impinges upon said substrate.

2. The invention as claimed in claim 1, in which said torch supplies said plasma to said chamber at a velocity in excess of Mach 1.

3. The invention as claimed in claim 1, in which a source of electrical power is connected between said torch and said substrate, to supply additional electrical energy to said plasma while it travels toward said substrate.

4. The invention as claimed in claim 1, in which said powder-supply means recited in clause (d) comprises at least one tube supplied by a source of spray powder and carrier gas, said tube being adapted to conduct said spray powder and carrier gas to plasma generated by said torch, and in which said preheating means recited in clause (e) comprises means to effect heating of said spray powder while in said tube.

5. The invention as claimed in claim 4, in which said means to heat said spray powder while in said tube comprises means to effect heating of said tube.

6. The invention as claimed in claim 5, in which said tube is electrically conductive, and in which said means to effect heating of said tube comprises means to effect flow of heating current through said tube.

7. The invention as claimed in claim 6, in which said tube is formed of metal having a relatively high electrical resistance.

8. The invention as claimed in claim 1, in which said powder supply means recited in clause ((1) comprises a plurality of sources of spray powder and carrier gas, and means to effect simultaneous flow of powder and gas from said sources to the plasma generated by said torch.

9. The invention as claimed in claim 1, in which said plasma-jet torch comprises a back electrode and a nozzle electrode, means to maintain a high-current electric arc between said back and nozzle electrodes, and means to supply arc gas to the space between said electrodes for heating by said arc and passing through said nozzle electrode into said chamber in the form of hightemperature plasma. 1

10. The invention as claimed in claim 9, in which said means to effect preheatingof said spray powder effects supplying of said spray powder to the interior of said torch.

11. The invention as claimed in claim 10, in which said means to effect preheating of said spray powder effects supplying of said spray powder to the nozzle passage through said nozzle electrode, and comprises at least one powder-introduction passage communicating transversely with said nozzle passage.

12. The invention as claimed in claim 11, in which said powder-introduction passage delivers powder to said nozzle passage in a generally upstream direction relative to the direction of plasma flow through said nozzle passage.

13. The invention as claimed in claim 12, in which said nozzle passage includes a throat and an end portion diverging from said throat to an exit, said powder introduction passage including a section extending along said diverging end portion and communicating with said throat. 7

14. Apparatus for effecting spray-coating of a powder onto a substrate, which comprises:

a. an electrical plasma torch comprising a back electrode and a nozzle electrode having a nozzle passage,

b. means to maintain a high-current electric arc between said electrodes,

c. means to introduce arc gas into the space between said electrodes, whereby said gas is heated by said arc and then passes outwardly through the nozzle passage of said nozzle electrode,

d. means to introduce spray powder into said nozzle passage, said spray powder means comprising at least one powder-introduction passage extending through the side wall of said nozzle pasage, and

e. means to effect a substantial amount of preheating of said spray powder prior to the time it passes through said nozzle passage wall and into the hot gas in said nozzle passage, said hot gas combining with said preheating means to achieve a highly effective heating of said spray powder.

15. The invention as claimed in claim 14, in which said preheating means comprises a powder tube communicating with said powder-introduction passage, said powder tube being formed of metal having a substantial electrical resistance, and means to pass a high current through said powder tube to heat the same and thus the spray powder flowing therethrough.

16. The invention as claimed in claim 15, in which said means to introduce spray powder further comprises means to introduce spray powder and carrier gas into said powder tube.

17. Apparatus for effecting spray-coating of a powder onto a substrate, which comprises:

a. an electrical plasma torch comprising a back electrode and a nozzle electrode having a nozzle passage,

b. means to maintain a high-current electric are between said electrodes,

c. means to introduce arc gas into the space between said electrodes, whereby said gas is heated by said are and then passes outwardly through the nozzle passage of said nozzle electrode,

d. means to introduce spray powder into said nozzle passage, said spray powder means comprising at least one powder-introduction passage extending through the side wall of said nozzle passage, and

e. means to effect a substantial amount of preheating of said spray powder prior to the time it passes through said nozzle passage wall and into the hot gas in said nozzle passage,

said hot gas combining with said preheating means to achieve a highly effective heating of said spray powder, said preheating means comprising a powder tube communicating with said powderintroduction passage, said powder tube being formed of metal having a substantial electrical resistance, and means to pass a high current through said powder tube to heat the same and thus the spray powder flowing therethrough, said means to pass current through said powder tube comprising a powder-preheat power source one terminal of which is connected to said tube and the other terminal of which is connected to said nozzle electrode, and means to electrically connect said nozzle electrode to said powder tube.

18. The invention as claimed in claim 17 in which said means to maintain a high-current electric are between said electrodes recited in clause (b) comprises a DC power source the negative terminal of which is connected to said back electrode and the positive terminal of which is connected to said nozzle electrode, and in which said powder preheat power source comprises a second DC power source the negative terminal of which is connected to said tube and the positive terminal of which is connected to said nozzle electrode.

19. The invention as claimed in claim 17, in which said means to connect said nozzle electrode to said powder tube comprises a split electrically conductive clamp ring connected to said powder tube, and means to clamp said ring on said nozzle electrode.

20. A method of applying a spray-coating onto a substrate, which comprises:

a. providing an environmental chamber,

b. reducing the pressure in said chamber to a value far below atmospheric pressure,

c. directing an electrically generated plasma jet of high-temperature gas into said chamber at a gas velocity in the range of Mach 1 to Mach 5,

d. entraining particles of spray material in said gas e. causing said jet and said spray material to impinge against a substrate to be coated, and

f. preheating said particles.

21. The invention as claimed in claim 20, in which said method further comprises causing the pressure in said chamber to be below one-half atmosphere.

22. The invention as claimed in claim 21, in which said method further comprises causing the pressure in said chamber to be below 40 mm of mercury.

23. The invention as claimed in claim 20, in which said method further comprises so adjusting the temperature of said jet and other parameters that said particles are'soft, but not molten, at the instant when they impinge against said substrate. I

24. The invention as claimed in claim 20, in which said step of preheating comprises effecting preheating of said particles of spray material prior to said entraining step recited in clause (d).

25. The'invention as claimed in claim 20, in which said method further comprises passing an electrict current through said jet to said substrate prior to commencement of said entraining step recited in clause (d), whereby to preheat said substrate.

26. The invention as claimed in claim 20, in which said method further comprises directing said jet against said substrate after cessation of said entraining step recited in clause (d), whereby to post-anneal said coated substrate.

27. The invention as claimed in claim 20, in which said method further comprises generating said gas jet by means of a high-current electrical plasma-jet torch.

28. The invention as claimed in claim 27, in which said substrate is electrically conductive, and including the step of passing a largeelectrical transfer current throughsaid jet by means of a circuit which includes said substrate.

29. The invention as claimed in claim 28, in which said transfer current is caused to be sufficiently large to effect localized fusion of said substrate.

30. A method of spray-coating a substrate, which comprises:

a. providing an electrical plasma-jet spray torch comprising a back electrode and a nozzle electrode,

b. maintaining a high-current electric are between said electrodes,

c. passing gas between said electrodes and then through the nozzle passage in said nozzle electrode, whereby said gas is heated by said arc and emanates from said nozzle passage in the form of a high-temperature, high-velocity jet,

(1. preheating particles of spray material and then causing said preheated particles to be entrained in said jet, and g e. directing said jet containing said particles against a substrate, whereby said particles adhere to said substrate and form a coating thereon.

31. The invention as claimed in claim 30, wherein said step of preheating particles and cuasing them to be entrained includes the step of flowing the particles through a heating tube and then through the wall of the nozzle electrode in a direction that extends rearwardly at an acute angle relative to the inner surface of said nozzle passage.

32. The invention as claimed in claim 30, in which said method further comprises so regulating. the amount of said preheating effected by step (d), and so regulating the temperature of said jet, that said particles are soft but not liquid at the instant of impingement against said substrate.

33. The invention as claimed in claim 30, in which said method further comprises causing the velocity of the gas in said jet to have a velocity in the range of Mach 1 to Mach 5.

34. The invention as claimed in claim 33, in which said method further comprises causing said substrate to be disposed, during performance of said spray-coating method, in an environmental chamber the pressure in which is only a small fraction of atmospheric pressure.

material into said gas through the wall of said nozzle passage, and effecting said preheating by passing said spray material through a hot conduit which communicates with said nozzle passage.

36. A method of forming a dense, high-purity coating on a substrate, which comprises:

a. providing an environmental chamber,

b. reducing the pressure in said chamber to a value far below atmospheric pressure,

c. disposing a substrate in said chamber,

d. employing anelectrical plasma-jet spray torch to generate a jet of high-temperature gas in said chamber,

e. causing the gas velocity in said jet to be in the range of Mach 1 to Mach 5,

f. effecting preheating of a spray powder,

g. introducing said preheated spray powder into said jet for acceleration thereby, and

h. directing said powder-containing jet against said substrate to thus cause said spray powder to coat said substrate.

37. The invention as claimed in claim 36, in which said method further comprises causing the pressure in said chamber to be below one half atmosphere, introducing said spray powder into said jet by passing said powder and a carrier gas through the wall of the nozzle passage in said spray torch, and preheating said spraypowder by passing the same through a hot elongated tube prior to said passing of said powder through the wall of said nozzle passage.

38. The invention as claimed in claim 37, in which said method further comprises passing a large electric current through said jet between said torch and said substrate.

39. The invention as claimed in claim 38, in which said method further comprises employing said large electric current to preheat said substrate prior to said introduction of spray powder, and employing said jet to post-anneal said substrate subsequent to cessation of said introduction of spray powder.

40. A method of applying a spray coating onto a substrate which comprises:

a. establishing and maintaining an electrically generated plasma arc,

b. expanding the plasma are within and projecting it from a nozzle as a super-sonic stream at a velocity in the range of Mach 1 to Mach 5,

c. entraining preheated particles of spray material in said plasma stream before the plasma is projected from the nozzle,

d. causing said plasma stream and spray material to impinge against the substrate to be coated, and

e. heating said spray material during its flight from said nozzle to said substrate by passing an electric current from the nozzle to the substrate through said spray material.

41. The method of claim 40 including the step of heating said spray material before it is injected into said plasma stream.

42. The method of claim 41 including the steps of providing an environmental chamber, reducing and maintaining the pressure in said chamber to a value far below atmospheric pressure, mounting the substrate to be coated within said chamber, and projecting said super-sonic plasma stream with the particles of spray material entrained therein into said chamber to impinge upon said substrate.

43. The method of spray coating a substrate with high velocity particles comprising the steps of a. establishing and maintaining an electrically generated plasma arc,

b. expanding said plasma are as a plasma stream from the throat to the exit of a super-sonic nozzle having a ratio of exit area to throat area sufficient to achieve a plasma stream exit velocity between Mach 1 and Mach 5,

c. injecting into said plasma stream a stream of particles to be projected, said particles having a diameter not greater than about 44 microns, and

d. heating said particles to a temperature near to but less than their melting point, said last-mentioned step comprising the step of applying additional heat to said particles before heat is imparted thereto by said plasma stream.

44. The method of claim 43 including the step of heating said particles in flight by passing an electrical current from said nozzle to a substrate to be coated by said particles, said electrical current passing through said super-sonic plasma stream.

45. The method of claim 44 wherein said step of passing an electrical current from said nozzle to said substrate is initiated before the step of injecting a stream of particles into the plasma stream so that the substrate is preheated before the particles impinge thereon. 

1. Apparatus for effecting spray-coating of a substrate, which comprises: a. means to define an environmental chamber, b. means to reduce the pressure in said chamber to a value below atmospheric pressure, c. an electrical plasma-jet torch positioned to direct a jet of plasma into said chamber, against a substrate disposed in said chamber, d. means to supply spray powder to said plasma jet for heating therein and subsequent impingment against said substrate, and e. means to effect preheating of said spray powder to an elevated temperature prior to the time said spray powder reaches said plasma, said preheating means cooperating with said plasma-jet torch to effect a high degree of heating of said powder prior to the time that said powder impinges upon said substrate.
 2. The invention as claimed in claim 1, in which said torch supplies said plasma to said chamber at a velocity in excess of Mach
 1. 3. The invention as claimed in claim 1, in which a source of electrical power is connected between said torch and said substrate, to supply additional electrical energy to said plasma while it travels toward said substrate.
 4. The invention as claimed in claim 1, in which said powder-supply means recited in clause (d) comprises at least one tube supplied by a source of spray powder and carrier gas, said tube being adapted to conduct said spray powder and carrier gas to plasma generated by said torch, and in which said preheating means recited in clause (e) comprises means to effect heating of said spray powder while in said tube.
 5. The invention as claimed in claim 4, in which said means to heat said spray powder while in said tube comprises means to effect heating of said tube.
 6. The invention as claimed in claim 5, in which said tube is electrically conductive, and in which said means to effect heating of said tube comprises means to effect flow of heating current through said tube.
 7. The invention as claimed in claim 6, in which said tube is formed of metal having a relatively high electrical resistance.
 8. The invention as claimed in claim 1, in which said powder supply means recited in clause (d) comprises a plurality of sources of spray powder and carrier gas, and means to effect simultaneous flow of powder and gas from said sources to the plasma generated by said torch.
 9. The invention as claimed in claim 1, in which said plasma-jet torch comprises a back electrode and a nozzle electrode, means to maintain a high-current electric arc between said back and nozzle electrodes, and means to supply arc gas to the space between said electrodes for heating by said arc and passing through said nozzle electrode into said chamber in the form of high-temperature plasma.
 10. The invention as claimed in claim 9, in which said means to effect preheating of said spray powder effects supplying of said spray powder to the interior of said torch.
 11. The invention as claimed in claim 10, in which said means to effect preheating of said spray powder effects supplying of said spray powder to the nozzle passage through said nozzle electrode, and comprises at least one powder-introduction passage communicating transversely with said nozzle passage.
 12. The invention as claimed in claim 11, in which said powder-introduction passage delivers powder to said nozzle passage in a generally upstream direction relative to the direction of plasma flow throUgh said nozzle passage.
 13. The invention as claimed in claim 12, in which said nozzle passage includes a throat and an end portion diverging from said throat to an exit, said powder introduction passage including a section extending along said diverging end portion and communicating with said throat.
 14. Apparatus for effecting spray-coating of a powder onto a substrate, which comprises: a. an electrical plasma torch comprising a back electrode and a nozzle electrode having a nozzle passage, b. means to maintain a high-current electric arc between said electrodes, c. means to introduce arc gas into the space between said electrodes, whereby said gas is heated by said arc and then passes outwardly through the nozzle passage of said nozzle electrode, d. means to introduce spray powder into said nozzle passage, said spray powder means comprising at least one powder-introduction passage extending through the side wall of said nozzle pasage, and e. means to effect a substantial amount of preheating of said spray powder prior to the time it passes through said nozzle passage wall and into the hot gas in said nozzle passage, said hot gas combining with said preheating means to achieve a highly effective heating of said spray powder.
 15. The invention as claimed in claim 14, in which said preheating means comprises a powder tube communicating with said powder-introduction passage, said powder tube being formed of metal having a substantial electrical resistance, and means to pass a high current through said powder tube to heat the same and thus the spray powder flowing therethrough.
 16. The invention as claimed in claim 15, in which said means to introduce spray powder further comprises means to introduce spray powder and carrier gas into said powder tube.
 17. Apparatus for effecting spray-coating of a powder onto a substrate, which comprises: a. an electrical plasma torch comprising a back electrode and a nozzle electrode having a nozzle passage, b. means to maintain a high-current electric arc between said electrodes, c. means to introduce arc gas into the space between said electrodes, whereby said gas is heated by said arc and then passes outwardly through the nozzle passage of said nozzle electrode, d. means to introduce spray powder into said nozzle passage, said spray powder means comprising at least one powder-introduction passage extending through the side wall of said nozzle passage, and e. means to effect a substantial amount of preheating of said spray powder prior to the time it passes through said nozzle passage wall and into the hot gas in said nozzle passage, said hot gas combining with said preheating means to achieve a highly effective heating of said spray powder, said preheating means comprising a powder tube communicating with said powder-introduction passage, said powder tube being formed of metal having a substantial electrical resistance, and means to pass a high current through said powder tube to heat the same and thus the spray powder flowing therethrough, said means to pass current through said powder tube comprising a powder-preheat power source one terminal of which is connected to said tube and the other terminal of which is connected to said nozzle electrode, and means to electrically connect said nozzle electrode to said powder tube.
 18. The invention as claimed in claim 17 in which said means to maintain a high-current electric arc between said electrodes recited in clause (b) comprises a DC power source the negative terminal of which is connected to said back electrode and the positive terminal of which is connected to said nozzle electrode, and in which said powder preheat power source comprises a second DC power source the negative terminal of which is connected to said tube and the positive terminal of which is connected to said nozzle electrode.
 19. The invention as claimed in claim 17, in which said means to connect sAid nozzle electrode to said powder tube comprises a split electrically conductive clamp ring connected to said powder tube, and means to clamp said ring on said nozzle electrode.
 20. A method of applying a spray-coating onto a substrate, which comprises: a. providing an environmental chamber, b. reducing the pressure in said chamber to a value far below atmospheric pressure, c. directing an electrically generated plasma jet of high-temperature gas into said chamber at a gas velocity in the range of Mach 1 to Mach 5, d. entraining particles of spray material in said gas jet, e. causing said jet and said spray material to impinge against a substrate to be coated, and f. preheating said particles.
 21. The invention as claimed in claim 20, in which said method further comprises causing the pressure in said chamber to be below one-half atmosphere.
 22. The invention as claimed in claim 21, in which said method further comprises causing the pressure in said chamber to be below 40 mm of mercury.
 23. The invention as claimed in claim 20, in which said method further comprises so adjusting the temperature of said jet and other parameters that said particles are soft, but not molten, at the instant when they impinge against said substrate.
 24. The invention as claimed in claim 20, in which said step of preheating comprises effecting preheating of said particles of spray material prior to said entraining step recited in clause (d).
 25. The invention as claimed in claim 20, in which said method further comprises passing an electrict current through said jet to said substrate prior to commencement of said entraining step recited in clause (d), whereby to preheat said substrate.
 26. The invention as claimed in claim 20, in which said method further comprises directing said jet against said substrate after cessation of said entraining step recited in clause (d), whereby to post-anneal said coated substrate.
 27. The invention as claimed in claim 20, in which said method further comprises generating said gas jet by means of a high-current electrical plasma-jet torch.
 28. The invention as claimed in claim 27, in which said substrate is electrically conductive, and including the step of passing a large electrical transfer current through said jet by means of a circuit which includes said substrate.
 29. The invention as claimed in claim 28, in which said transfer current is caused to be sufficiently large to effect localized fusion of said substrate.
 30. A method of spray-coating a substrate, which comprises: a. providing an electrical plasma-jet spray torch comprising a back electrode and a nozzle electrode, b. maintaining a high-current electric arc between said electrodes, c. passing gas between said electrodes and then through the nozzle passage in said nozzle electrode, whereby said gas is heated by said arc and emanates from said nozzle passage in the form of a high-temperature, high-velocity jet, d. preheating particles of spray material and then causing said preheated particles to be entrained in said jet, and e. directing said jet containing said particles against a substrate, whereby said particles adhere to said substrate and form a coating thereon.
 31. The invention as claimed in claim 30, wherein said step of preheating particles and cuasing them to be entrained includes the step of flowing the particles through a heating tube and then through the wall of the nozzle electrode in a direction that extends rearwardly at an acute angle relative to the inner surface of said nozzle passage.
 32. The invention as claimed in claim 30, in which said method further comprises so regulating the amount of said preheating effected by step (d), and so regulating the temperature of said jet, that said particles are soft but not liquid at the instant of impingement against said substrate.
 33. The invention as claimed in claim 30, in which said method further comprises causing the Velocity of the gas in said jet to have a velocity in the range of Mach 1 to Mach
 5. 34. The invention as claimed in claim 33, in which said method further comprises causing said substrate to be disposed, during performance of said spray-coating method, in an environmental chamber the pressure in which is only a small fraction of atmospheric pressure.
 35. The invention as claimed in claim 30, in which said method further comprises introducing said spray material into said gas through the wall of said nozzle passage, and effecting said preheating by passing said spray material through a hot conduit which communicates with said nozzle passage.
 36. A method of forming a dense, high-purity coating on a substrate, which comprises: a. providing an environmental chamber, b. reducing the pressure in said chamber to a value far below atmospheric pressure, c. disposing a substrate in said chamber, d. employing an electrical plasma-jet spray torch to generate a jet of high-temperature gas in said chamber, e. causing the gas velocity in said jet to be in the range of Mach 1 to Mach 5, f. effecting preheating of a spray powder, g. introducing said preheated spray powder into said jet for acceleration thereby, and h. directing said powder-containing jet against said substrate to thus cause said spray powder to coat said substrate.
 37. The invention as claimed in claim 36, in which said method further comprises causing the pressure in said chamber to be below one half atmosphere, introducing said spray powder into said jet by passing said powder and a carrier gas through the wall of the nozzle passage in said spray torch, and preheating said spray powder by passing the same through a hot elongated tube prior to said passing of said powder through the wall of said nozzle passage.
 38. The invention as claimed in claim 37, in which said method further comprises passing a large electric current through said jet between said torch and said substrate.
 39. The invention as claimed in claim 38, in which said method further comprises employing said large electric current to preheat said substrate prior to said introduction of spray powder, and employing said jet to post-anneal said substrate subsequent to cessation of said introduction of spray powder.
 40. A method of applying a spray coating onto a substrate which comprises: a. establishing and maintaining an electrically generated plasma arc, b. expanding the plasma arc within and projecting it from a nozzle as a super-sonic stream at a velocity in the range of Mach 1 to Mach 5, c. entraining preheated particles of spray material in said plasma stream before the plasma is projected from the nozzle, d. causing said plasma stream and spray material to impinge against the substrate to be coated, and e. heating said spray material during its flight from said nozzle to said substrate by passing an electric current from the nozzle to the substrate through said spray material.
 41. The method of claim 40 including the step of heating said spray material before it is injected into said plasma stream.
 42. The method of claim 41 including the steps of providing an environmental chamber, reducing and maintaining the pressure in said chamber to a value far below atmospheric pressure, mounting the substrate to be coated within said chamber, and projecting said super-sonic plasma stream with the particles of spray material entrained therein into said chamber to impinge upon said substrate.
 43. The method of spray coating a substrate with high velocity particles comprising the steps of a. establishing and maintaining an electrically generated plasma arc, b. expanding said plasma arc as a plasma stream from the throat to the exit of a super-sonic nozzle having a ratio of exit area to throat area sufficient to achieve a plasma stream exit velocity between Mach 1 and Mach 5, c. injecting into said plasma sTream a stream of particles to be projected, said particles having a diameter not greater than about 44 microns, and d. heating said particles to a temperature near to but less than their melting point, said last-mentioned step comprising the step of applying additional heat to said particles before heat is imparted thereto by said plasma stream.
 44. The method of claim 43 including the step of heating said particles in flight by passing an electrical current from said nozzle to a substrate to be coated by said particles, said electrical current passing through said super-sonic plasma stream.
 45. The method of claim 44 wherein said step of passing an electrical current from said nozzle to said substrate is initiated before the step of injecting a stream of particles into the plasma stream so that the substrate is preheated before the particles impinge thereon. 